1. Field Of The Invention
This invention relates to a process for treating an aqueous ore slurry of a carbonaceous precious metal-containing ore in a cyanide leach, carbon-in-pulp system to inhibit mercury from being adsorbed onto activated carbon. This invention more particularily relates to a process for treating the aqueous ore slurry of a carbonaceous precious metal-containing ore by the addition of sulfide compounds subsequent to completion of an oxidation step and prior to a cyanide leach, carbon-in-pulp adsorption circuit.
2. Description of the Prior Art
Precious metal-containing ores such as gold-containing ores often contain mercury compounds as impurities. The presence of these mercury compounds is undesirable particularly in gold recovery plants using cyanide leach, carbon-in-pulp systems. In such systems both gold and mercury are leached from the ore in the presence of the activated carbon which is in the cyanide leach slurry or pulp. The simultaneous leaching of mercury and gold causes some mercury to be adsorbed onto the activated carbon thereby reducing the surface area of the activated carbon available to adsorb the gold. The competition of mercury and gold during the activated carbon adsorption process increases the amount of activated carbon required to adsorb the leached gold. Another problem associated with the presence of mercury in a gold recovery process is that any mercury not removed during the initial gold processing operation can be present in the intermediate gold product. The presence of mercury in the intermediate gold product can increase the cost of refining the gold concentrate.
U.S. Pat. No. 4,289,532 to Matson, herein incorporated by reference, is illustrative of a gold recovery process using chlorination in a cyanide leach, carbon-in-pulp system. In Matson the recovery of gold from refractory carbonaceous gold-containing ores is accomplished by subjecting an aqueous carbonaceous slurry to an oxidation step which includes an oxygenation step and/or a chlorination step. The oxidized aqueous slurry is simultaneously contacted with a cyanide leach solution and granular activated carbon. The gold contained in the ore is leached from the ore and then adsorbed by the activated carbon. The absorbed gold is then "stripped" or desorbed from the gold-loaded activated carbon. This desorption of gold from the activated carbon is accomplished by the use of a cyanide stripping solution. The Matson process has proven to be a commercially successful gold extraction process. The difficulties encountered with mercury contamination decrease the efficiency of the Matson process.
One effective method for removing mercury compounds from gold recovery systems is to retort the gold concentrate. Mercury is distilled from the gold concentrate during the retorting operation and is collected in a condenser from which the mercury is sold or disposed of in an environmentally safe manner. A retorting operation is costly because of the expensive equipment and large amounts of energy required to conduct the method.
In gold recovery plants utilizing cyanide leach recovery systems, mercury compounds can be precipitated and removed from cyanide solutions by the addition of metal sulfides to the cyanide solutions. U.S. Pat. No. 4,256,707 to Flynn et al. discloses a process for selectively removing mercury from gold-cyanide solutions. A mercuric sulfide precipitate is formed by adding Ag.sub.2 S, ZnS, or FeS to the gold-cyanide solution. The Flynn patent discloses that when sulfide is added to a gold-cyanide solution in an amount less than 1.5 times the stoichiometric amount required to precipitate mercury, the formation of insoluble mercuric sulfide (HgS) is distinctly favored over the formation of other metal sulfides. This selective precipitation of mercuric sulfide (HgS) is performed in a cyanide solution at ambient temperature and atmospheric pressure. The cyanide solutions described in Flynn are typical of those solutions obtained after solid ore particles have been separated from the aqueous phase of ore slurries and after cyanide has been added to the solution. The Flynn patent fails to disclose a pretreatment for precipitating mercury from aqueous ore slurries prior to the removal of the "pulp", which is defined as the solid ore particles in the liquor, or a process that is readily adaptable to cyanide leach, carbon-in-pulp systems.
The industry for extracting gold or other precious metals from carbonaceous ores has increasingly used chlorination processes to oxidize the carbonaceous materials in the ores. This makes the ores more amendable to cyanide leaching. The increase in the number of gold plants using chlorination processes to extract gold has increased the industry's concern over mercury contamination of activated carbon. The industry lacks a process for inhibiting mercury adsorption onto the activated carbon used in cyanide leach, carbon-in-pulp precious metal extraction systems.